Mononuclear phagocytes constitute an important element of the host's defenses against the development and spread of neoplasia. When macrophages are activated for cytolysis, they effectively and selectively lyse tumor cells, but the precise mechanisms responsible for this target recognition and destruction remain to be defined. We have established that the activation of macrophages alters their function so that the macrophages selectively bind neoplastic targets and secrete a potent cytolytic factor. The cytolytic factor, which is related to a specific neutral protease secreted only by activated macrophages, selectively lyses neoplastic cells. The ability of macrophages to mediate cytolysis requires both the ability to bind neoplastic targets and to secrete cytolytic factor; each of these functions appears to be a separate and independently regulated capacity. We hypothesize that cytolysis by activated macrophages is a multistep event, encompassing an initial step of macrophage-target binding and, subsequently, a step of target attack characterized by secretion of cytolytic substances, including cytolytic factor from the macrophages. We propose here to test critically this hypothesis, establish the steps in macrophage-mediated cytolysis, determine the basic nature of the binding and target attack steps and establish the molecular character of cytolytic factor. These studies will define and characterize the means by which activated macrophages affect cytolysis of neoplastic cells. Elucidation of this problem will amplify our understanding of how the mononuclear phagocyte system protects the host from foreign intruders.